I'm glad your parents love you enough to get involved and see what you are playing. Kudos to them, they are good Christian parents.
When I first picked up GW a year and a half ago, I was also concerned about magic, Necros and such. For that reason, I picked a Ranger as my first character and Warrior as my second. Since then I've broadened into creating a Ritualist, Assassin, Ele and Derv (my current favorite).
The magic in GW isn't any different than what you'd see in Narnia and Lord of the Rings. If your parents think those are of Satan, then you have a problem.
I'm probably your parents age, if not older. I don't have a problem with my daughter playing GW and have been encouraging my son to play it as well. As others have said, show your parents the large and supportive Christian community here. If that doesn't sway them, then you've done all you can. Whatever their final decision, you must honor and obey them.
On another note, encourage them to play the game! The oldest person I personally know that plays is in her late 60's and has maxed out a bunch of titles. There are a lot of people in their 30's, 40's and 50's. It's a fun game for almost all ages.